1. Field of the Invention
The present invention relates to a radio communication device preferably for use in an RFID system or a near field radio communication system.
2. Description of the Related Art
In general, a planar coil antenna is used in HF-band RFID such as NFC (Near Field Communication) implemented in a mobile terminal.
Meanwhile, a radio communication device such as a recent cellular phone terminal has been thinned. In order to deal with insufficiency of strength due to thinning, or from the standpoint of design, a resin casing is subjected to metal plating and a metal casing is used in an increasing number of applications.
However, in the case where metal is used for a casing, since a planar coil antenna included in the terminal faces a metal surface, an induced current (eddy current) flows through the metal surface so as to cancel a signal current in the planar coil. Thus, the planar coil antenna is shielded by the metal surface, thereby disabling communication with a counterpart device.
An invention regarding an antenna device for the purpose of solving the problem caused by the eddy current is disclosed in Japanese Unexamined Patent Application Publication No. 2009-4857. In the antenna device described in Japanese Unexamined Patent Application Publication No. 2009-4857, a loop antenna is disposed parallel to and near a plate conductor, and a slit is formed in the plate conductor so as to perpendicularly traverse the arc of the loop antenna.
In the antenna device described in Japanese Unexamined Patent Application Publication No. 2009-4857, although a loop-shaped mirror image current which is a mirror image with respect to an antenna current flowing through the loop antenna flows through the plate conductor, if the slit of the plate conductor is formed so as to transverse the mirror image current, the mirror image current hits an edge portion of the slit to be divided into two directions, and the divided current flows through the peripheral edge of the plate conductor in the direction opposite to the mirror image current. This is used to reduce cancellation of a magnetic field of the antenna current caused due to a magnetic field of the mirror image current.
In the antenna device shown in Japanese Unexamined Patent Application Publication No. 2009-4857, the slit is formed in the outer periphery of the plate conductor, so that a mirror image current flows along the edge of the plate conductor. Thus, the mechanical strength of the plate conductor decreases, and, for example, it is difficult to apply the structure of the above-described plate conductor to a metal casing, without decreasing the mechanical strength. In addition, when the antenna device shown in Japanese Unexamined Patent Application Publication No. 2009-4857 is applied to a metal casing of the radio communication device, great restrictions are imposed in terms of external design.